One of the enduring critiques of Donald Trump from conservative Republicans throughout the primary process has been that the businessman’s cocksure temperament and draconian immigration policy proposals, though popular among certain segments of the Republican primary electorate, will all but ensure his defeat in a general election campaign against Hillary Clinton, the presumptive Democratic Party nominee. I have levied this criticism against Mr. Trump in the past; after all, his approval ratings among women and Hispanics are awful and are unlikely to improve dramatically before voters cast their ballots in November, even if the businessman makes a concerted effort to soften his image within these critical demographic groups.

But since Donald Trump took his place as the presumptive Republican nominee, the long succession of general election polls which showed the real estate mogul trailing both of his potential rivals by a wide margin has been completely overturned. Before Mr. Trump’s decisive victory in Indiana’s primary on May 3rd forced Ted Cruz and John Kasich out of the Republican nomination fight, the GOP frontrunner was polling badly against both potential Democratic opponents. General election polls taken in the month of April showed Mr. Trump trailing Hillary Clinton and Bernie Sanders by an average of just about 10 and 12 percentage points, respectively. By contrast, in April 2012, Mitt Romney, who ultimately became the Republican nominee for that year’s presidential election, only lagged President Obama by an average of five points across seventeen general election polls.

But in the polls taken after his ascendance to the status of presumptive nominee, Donald Trump has gained ground rapidly. Though Bernie Sanders still trounces him by an average of 9.4 percent, Mr. Trump has closed what was a double-digit gap between him and Mrs. Clinton to a mere 1.2 point margin. Taking only the polls conducted in the last week, the businessman pulls ahead of Mrs. Clinton, albeit by less than one percent on average. Yet remarkably, these same polls show Mr. Trump to suffer still from the same likability problems which his critics and opponents have identified. According to a Fox News poll conducted this week, Mr. Trump is disliked by nearly three-fifths of the national electorate, yet he still manages to defeat Mrs. Clinton by three points. What gives?

First, though voters still dislike Mr. Trump immensely, they like him more now than they have in the past. Two months ago, the percentage of voters which view him negatively reached 65 percent, a record high for the duration of Fox News’s general election polling so far in this election. In the most recent survey, this number declined to 56 percent. This change could be reflecting a number of factors, including Mr. Trump’s move toward more moderate policy stances and less strident rhetoric as he shifts toward a general election strategy.

Second, Republicans are beginning to dutifully unite behind Mr. Trump as their party’s nominee. According to Gallup, which has tracked the businessman’s favorability among registered Republicans and Republican-leaning independents since July 2015, conservative voters are now more united behind him than ever, with two-thirds viewing him favorably now. Though many Republicans remain dissatisfied with Mr. Trump as their party’s nominee, they still largely plan to support him, and the once powerful #NeverTrump movement is now greatly diminished in strength and resolve.

Third, voters view Hillary Clinton as a poor alternative to Mr. Trump. According to this week’s Fox News poll, over three-fifths of the electorate—61 percent—view her unfavorably, while only 56 percent view Donald Trump in this way. Among independents, which have been a decisive force in the past several presidential contests, Mrs. Clinton’s favorability rating is atrocious, with a whopping 51-point gap between voters who view her favorably (23 percent) and those who do not (74 percent). Independents also dislike Mr. Trump, but his net favorability (-17 percent) is much better than Clinton’s. The former secretary of state also fares poorly on questions of trustworthiness, as many voters view her as the epitome of a slick career politician. Nearly half of voters view her as more corrupt than Mr. Trump, including nearly a fifth of Democrats.

Fourth, it is becoming increasingly unclear whether Mrs. Clinton will manage to unite the Democratic Party behind her candidacy. Though she has received strong support from registered Democrats in several recent surveys, what really matters is whether she will be able to woo Bernie Sanders’s supporters—best characterized as liberals who do not identify as Democrats—to her cause. But as relations between Mrs. Clinton and Mr. Sanders have soured, her favorability rating among the Vermont senator’s following has cratered. According to a YouGov poll conducted early this month, 61 percent of Mr. Sanders’s supporters view Mrs. Clinton unfavorably, and only 55 percent said that they would vote for Clinton over Trump in November. Of course, much can change between now and the November election, and we will undoubtedly see Sanders’s supporters shift at least slightly toward Hillary Clinton once she clinches the nomination. But unlike Mr. Trump, whose base of support largely consists of disaffected working-class people of independent political persuasions, Mrs. Clinton draws most of her support from older voters who identify themselves with the Democratic Party.

This difference could ultimately prove fatal for Clinton’s campaign. It has long been the case that party identification is one of the best predictors of how people will vote. As one study of voting behavior in the 2008 election explains, “party identification is a perceptual screen: a pair of partisan-tinted eyeglasses through which the voter views the political world.” As such, it goes without saying that Mrs. Clinton will command most of the vote from Democratic Party-affiliated voters in November. But this portion of the general electorate is one which virtually any Democratic nominee would win anyway simply by virtue of his/her party affiliation. The real battle is for moderates and independent voters, who are agents of transformation in an otherwise static political arena, yet these voters do not like Mrs. Clinton at all.

Donald Trump is much better positioned in this respect. Though many Republicans are uneasy about his candidacy, their loyalty toward GOP-affiliated candidates will pull them inexorably toward their party’s nominee. Meanwhile, Mr. Trump will hold onto his core group of supporters—the powerful bloc of independent blue-collar voters which his candidacy has energized—for the November election.

For months, I and many political pundits believed that Donald Trump would be a weak general election candidate. But as the general election matchup between Mrs. Clinton and Mr. Trump has solidified, it is becoming increasingly uncertain just who exactly is the underdog in this year’s fight for the highest office in the land.